The present invention relates to a main steam isolation valve particularly of a boiling water reactor (BWR) plant for suppressing oscillation of a valve disk of the steam isolation valve.
Generally, in a BWR plant, a reactor pressure vessel is directly connected to a steam turbine through a plurality of main steam pipes, and first and second main steam isolation valves are incorporated to each of the main steam pipes inside and outside of the reactor container. The reactor pressure vessel is isolated as occasion demands by closing these main steam isolation valves.
The steam used in the steam turbine is condensed into condensate in a condenser and the condensate is then returned to the reactor pressure vessel.
In a conventional main steam isolation valve, inlet and outlet end portions of a valve body are connected to each of the main steam pipes and a valve disk is accommodated in the valve body in an axially reciprocal manner. The valve disk is provided with a valve shaft which is inclined inwardly, in an installed state, by about 45.degree. with respect to the flow direction of the mainsteam thereby to reduce flow resistance. The valve shaft is connected at one end to a driving means to reciprocatingly move the valve disk thereby to open or close the fluid, i.e. steam, passage.
The reciprocating motion of the valve disk is guided with a central guide rib and a bilateral pair of side guide ribs inwardly projecting at rear side portions of the valve body circumferentially apart from the central guide rib with a separation angle of about 120.degree. with each other.
In such a structure, when the valve disk is rested on a valve seat, the valve disk is fully closed and the valve disk is upwardly lifted thereby to fully open a valve port.
In the conventional structure of the main steam isolation valve, since the bottom portion of the valve disk is held, at its fully opened state, with substantially the half portion thereof being projected into the fluid passage, the projected bottom portion is exposed to the steam flow, thus being subjected to fluid pressure in the main steam flowing direction and a direction normal thereto. Because the fluid pressure is in proportion to two squares of the fluid velocity, the valve disk may be oscillated in case of high fluid velocity. When the valve disk is oscillated, the central guide rib is rubbed with the valve disk with each other, resulting in the wear thereof and hence causing the leakage of the steam even in the case of the fully closed state of the valve.
Furthermore, since the steam flow is prevented by the paired side guide ribs and the valve disk, the pressure is locally increased at the upstream side of the steam flow, and since a force for pressing the valve disk against the side guide rib sides, friction force between the guide ribs and the valve disk is increased, thus the degree of wear therebetween being also increased, resulting in the increased possibility of causing the leakage of the steam.